Maybe if you apply for the information and say you want to use it to
write this kind of tool, then Adobe don't grant permission. It's just
a thought, not inside knowledge. Also, maybe the format includes some
things that are not documented; perhaps the format documentation is
enough to read all image information from PSD files, but not enough to
write a new one that is valid. It also may be that the tools you
found are based on very old format information, not up to date with
current versions of Photoshop.
Adobe do have something called the "Graphics Server" which might be
able to do this.
Also, for lower volumes, you could use Photoshop automation to do your
task.
Aandi Inston